1. Field of the Invention
The present invention relates to a method of manufacturing a polytetrafluoroethylene (PTFE) tape of very high porosity, to a porous tape obtained thereby, and to use thereof, in particular in a coaxial cable.
2. Description of the Related Art
Porous PTFE tapes already exist having a density of 0.2 grams per cubic centimeter (g/cm.sup.3), which corresponds to a dielectric constant (.epsilon.) of about 1.09.
Such PTFE tapes are manufactured by conventional stretching methods that include a single and continuous stretching step at temperatures that are well below the melting temperature (freezing point) of PTFE. Such methods encounter a density limit at about 0.3 g/cm.sup.3. Thus, when stretching comes close to this value, the tape breaks.
To achieve lower densities, known methods use at least two successive stretching passes, that imply changes to the structure of the PTFE which goes from a state in which its degree of crystallinity is high to a state in which the amorphous fraction is preponderant.
This sudden change of state is caused by a sintering operation that is performed after the stretching operation and at a temperature close to the melting temperature (freezing point) of the raw PTFE tape.
The structure is locked in the amorphous state by a rapid cooling step.
It should be observed that the dielectric constant of PTFE is related to its porosity, its specific gravity, or its density by the equation .epsilon.=(1+d/3)/(1-d/12.6) where d is specific gravity.
In addition, the relationship between porosity P and specific gravity d (or density in g/cm.sup.3) is the following: EQU P=(1.1-d/2)/1.1
Consequently, one of the major uses of said tape lies in providing insulating coatings for cables, in particular coaxial cables.